Retainer-ring of cmp (chemical mechanical polishing) machine

ABSTRACT

A retainer ring is provided which may restrain the time taken for a break-in polish to a minimum. This retainer ring may be disposed inside of a holding head in a CMP apparatus which polishes a wafer chemically and mechanically; may have a ring shape so as to surround the periphery of the wafer; may press a polish surface of a polish pad; may be made of an engineering plastic material such as PPS; and may have a pressure surface for pressing the polish surface of the polish pad whose surface roughness is a center-line average roughness (Ra) of 0.2 μm or below.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional Patent Application No. 60/924,744, entitled Retainer-ring of CMP (Chemical Mechanical Polishing) Machine, filed on May 30, 2007, which is related to JP 2005-352821 filed Dec. 7, 2005, the entire specification, claims, and drawings of each of which is incorporated herein by reference.

BACKGROUND

1. Field

The present invention relates to a CMP (chemical mechanical polishing) apparatus which polishes a wafer (a body to be polished) chemically and mechanically. Particularly, it relates to a retainer ring which is provided on (attached to) the inside of a holding head of the CMP Apparatus and surrounds the periphery of the wafer.

2. Background

As a semiconductor device has more highly integrated and has performed better, its measurements in the horizontal directions (on the plane) have shortened and the structure in the vertical directions has been fined down and multi-layered. In order to realize such a fine and multi-layered structure, a semiconductor substrate (such as a silicon substrate) needs to have a high flatness (evenness). Hence, the flatness has to be heightened at the stage of a wafer, and in response to this demand, a CMP Apparatus is used.

This CMP Apparatus is configured by, for example: a rotary base; a polish pad disposed on this base; and a holding head which holds a wafer and presses it onto the polish pad; a slurry supply nozzle; and the like. The holding head is formed by: a retainer ring which surrounds the periphery of the wafer; for example, an elastic film which pushes (presses) the upper surface of the wafer; an air chamber which is enclosed with this elastic film, the retainer ring and a head body; an air supply path which supplies air for pressurization into this air chamber. The retainer ring surrounds the periphery of the wafer and prevents the wafer from popping out. It also presses the polish surface of the polish pad, and flattens and fines down (suits) the polish surface of the polish pad which polishes the wafer (e.g., refer to Patent Document 1, Japanese Patent Laid-Open No. 11-226865).

SUMMARY

Since such a retainer ring presses the polish pad, the retainer ring's pressure surface (surface for pressing the polish pad) itself is supposed to be polished and worn off by the polish pad. This requires that the retainer ring be regularly replaced with a new retainer ring. However, if the new retainer ring's pressure surface is rough, then a predetermined polishing performance cannot be obtained. Specifically, polish scrap may adhere to a wafer's surface to be polished, or scratches or the like can be produced thereon. In the case where the retainer ring has been exchanged, therefore, before a product wafer (a wafer which is manufactured as a product) begins to be polished, a break-in (preparatory) polish needs to be given using a newly-attached retainer ring. Specifically, this new retainer ring is attached to the holding head of the CMP Apparatus and dozens to hundreds of dummy wafers (break-in wafers) are polished. After it is ascertained that a suitable polishing performance has been obtained, for the first time, a polish is given to the product wafer. This break-in polishing requires a great deal of time and labor, thus lowering the production availability of wafers.

Therefore, it is an object of the present invention to provide a retainer ring which is capable of restraining the time taken for a break-in polish to the minimum.

In order to attain the above described object, a CMP-apparatus retainer ring according to claim 1 which, in a CMP apparatus that includes a polish pad disposed on a base and a holding head holding a wafer and pressing the wafer against the polish pad and that polishes the wafer chemically and mechanically: is provided inside of the holding head; has a ring shape so as to surround the periphery of the wafer; and presses the polish surface of the polish pad, characterized in that the surface roughness of a pressure surface for pressing the polish surface of the polish pad is a center-line average roughness of 0.2 μm or below.

The inventors have made experiments and studies repeatedly. Thereby, they have found out that in a retainer ring, if its pressure surface's surface roughness is set to a center-line average roughness of 0.2 μm or below, the time taken for a break-in polish can be restrained to the minimum. Hence, in the CMP-apparatus retainer ring according to claim 1, the time taken for a break-in polish can be kept down to the minimum. In other words, in a retainer ring, if its pressure surface's surface roughness is a center-line average roughness of 0.2 μm or below, a high polishing performance can be obtained from immediately after the retainer ring has been attached to the holding head of the CMP apparatus, or after a break-in polish has been given for a short time. Specifically, the polish pad's polish surface is flattened and fined down desirably (up to a high level) with the retainer ring's pressure surface. Thereby, polish scrap is far less likely to adhere to a wafer's surface to be polished, or scratches or the like are even less likely to be produced thereon. As a result, the time and labor taken for such a break-in polish can be kept down to the minimum. Effectively in practice, this makes it possible to enhance the production availability of the CMP apparatus. Besides, the time taken for a break-in polish comes to the minimum, so that polish scrap produced by giving the break-in polish to dummy wafers can be reduced, scratches on a wafer (a product wafer) by such polish scrap are still less likely, or impurities are much less likely to adhere to a wafer. Consequently, the production quality of a wafer becomes higher and more stable, thus decreasing the number of defective products and enhancing the productivity further.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a front view of a CMP apparatus showing a schematic configuration thereof;

FIG. 2 illustrates a schematic sectional view of a holding head of the CMP apparatus;

FIG. 3 illustrates a front view (a) and a bottom view (b) of a CMP-apparatus retainer;

FIG. 4 is a table showing the number of defects and other items after a break-in polish in accordance with each surface roughness of the pressure surface in the retainer ring; and

FIG. 5 is a log-log graph showing the relation between the pressure surface's surface roughness in the retainer ring and the number of defects after ten dummy wafers are polished.

DETAILED DESCRIPTION

The detailed description set forth below in connection with the appended drawings is intended as a description of various embodiments of the present invention and is not intended to represent the only embodiments in which the present invention may be practice. The detailed description includes specific details for the purpose of providing a thorough understanding of the present invention. However, it will be apparent to those skilled in the art that the present invention may be practiced without these specific details.

FIG. 1 is a front view of a CMP apparatus 1 according to an embodiment of the present invention, showing a schematic configuration thereof. This CMP apparatus 1 has a configuration equivalent to a CMP apparatus which is widely used in general, apart from a retainer ring 8 (described later). Herein, a detailed description is omitted. It includes: a base 2 which can be rotated; a polish pad 3 (such as a cloth) which is disposed on this base 2; a holding head 4; a slurry supply nozzle 5; and a dresser 6 (a dressing means). It polishes a wafer W chemically and mechanically.

The holding head 4 holds the wafer W and presses its surface W1 to be polished against the polish pad 3. It is designed to move on the polish pad 3 while rotating. This holding head 4 is, as shown in FIG. 2, provided with: a head body 7; the retainer ring 8 which is disposed under this head body 7; and an elastic film 9 which is located inside of this retainer ring 8 and presses an upper surface W2 of the wafer W. Then, an air chamber 10 is enclosed with the head body 7, the retainer ring 8 and the elastic film 9, and air for pressurization is supplied into this air chamber 10. Via the elastic film 9, the holding head 4 presses the wafer W onto the polish pad 3.

The retainer ring 8 surrounds the periphery of the wafer W and prevents the wafer W from popping out of the holding head 4. It also presses a polish surface 3 a of the polish pad 3, and flattens and fines down (suits) the polish surface 3 a (the part which comes into surface contact with the polished surface W1 of the wafer W) of the polish pad 3 which polishes the wafer W. Specifically, the polish surface 3 a of the polish pad 3 becomes so as that its flatness is low and the surface roughness is rough with using slurry 5 a (abrasive). Hence, the retainer ring 8 enhances this polish surface 3 a's flatness and lowers the surface roughness.

In consideration of chemical resistance, mechanical properties or the like, this retainer ring 8 is made of a PPS (polyphenylene sulfide, engineering plastic) material. It has, as shown in FIG. 3, a ring shape. In a pressure surface 8 a for pressing the polish surface 3 a of the polish pad 3, a plurality of slits 8 b are formed which are each shaped like a groove for allowing polish scrap to escape (discharge). In a back surface 8 c of the retainer ring 8 located at the back surface of the pressure surface 8 a, a plurality of screw inserts 8 d each of which has a female thread 8 e are inserted. In other words, such a screw insert 8 d is substantially cylindrical, and a male thread is formed in its external-circumference part and the female thread 8 e is formed in its internal-circumference part.

In this retainer ring 8, the pressure surface 8 a's surface roughness is set (the surface is polished) to a center-line average roughness (arithmetic average roughness, Ra) of 0.2 μm or below. This is obtained through repeated experiments and studies made by the inventors. In short, the inventors have found out that if its pressure surface's surface roughness is set to a center-line average roughness of 0.2 μm or below, the time taken for a break-in polish can be restrained to the minimum. FIG. 4 shows data which proves (verifies) this.

This data indicates the number of defects and other items after a break-in polish in accordance with each surface roughness (center-line average roughness Ra). Specifically, in this figure, “Defect number after ten dummy wafers (DN1)” indicates the number of defects which are detected on the tenth dummy wafer when polishing has been completed for ten dummy wafers (break-in wafers). Herein, the defect includes every defective element, such as foreign matter like polish scrap and a scratch. In the figure, “Dummy-wafer polish number (PN)” indicates the number of polished dummy wafers. Then, “Defect number after PN dummy wafers (DN2)” indicates the number of defects which are detected on the PNth dummy wafer when polishing has been completed for PN dummy wafers and which have a size (length) larger than 0.16 μm. Finally, “Scratch number among DN2 (SN)” indicates, among the defect number DN2, the number of scratches which have a size (length) larger than 1.0 μm.

The break-in polishing conditions for dummy wafers are given below:

Slurry: SS-25 (by Cabot Corporation);

Polish pad: IC1000/SUBA400 (by Rohm and Haas Company);

Polish time per dummy wafer: 60 seconds; and

Polishing pressure (pressing force) on dummy wafers; 210 hPa.

As is obvious from this data, in the retainer ring 8, if the pressure surface 8 a's surface roughness Ra is above 0.2 μm (if the surface roughness Ra is 0.4 μm or 0.3 μm), as shown in FIG. 5, the defect number DN1 after ten dummy wafers have been polished reaches a few hundred. In contrast, if the surface roughness Ra is 0.2 μM or below, it drops sharply to twenty or under. Besides, if the pressure surface 8 a's surface roughness Ra is rougher than 0.2 μm, the defect number DN2 after some hundreds of (PN) dummy wafers have been polished reaches about thirty. On the other hand, if the surface roughness Ra is equal to, or below, 0.2 μm, the defect number DN2 reaches twenty or under only by polishing ten dummy wafers. In addition, if the pressure surface 8 a's surface roughness Ra exceeds 0.2 μm, the scratch number SN among the defect number DN2 is approximately five to ten, while if the surface roughness Ra is 0.2 μm or below, there are hardly any scratches,

As described above, the polishing performance can be sharply enhanced when the surface roughness Ra is set to be equal to, or below, 0.2 μm which corresponds to the boundary (critical point/value) (see FIG. 5). Hence, in the retainer ring 8, if the pressure surface 8 a's surface roughness is set to a center-line average roughness of 0.2 μm or under, the time taken for a break-in polish can be restrained to the minimum.

The polishing process to set the center-line average roughness to 0.2 μm or below may involve subjecting the pressure surface 8 a to surface cutting in a lathe. The pressure surface 8 a may be shallowly cut and the rotation of the lathe slowed so that the pressure surface 8 a has a high flatness and a low surface roughness. In accordance with the retainer ring 8's material properties, including hardness, some other machining, such as grinding, may be conducted in place of lathe cutting. The pressure surface 8 a may then be subjected to polishing by a polishing apparatus, such as a lapping machine or a polishing machine. The pressure surface 8 a may be pressed against a rotating polishing pad, or vice versa, the pressure surface 8 a may be pressed against a polishing pad while rotating the retainer ring 8. In accordance with the retainer ring 8's material properties, hardness or size, for example, a kind of slurry (abrasive) or the like may be supplied to the polish pad. The polishing conditions may be regulated by controlling some of the variables, including pressure applied to the pressure surface 8 a by the polish pad, the rotational speed of the pad in relation to the ring 8, or the amount of slurry.

In other words, if the pressure surface 8 a's surface roughness Ra is 0.2 μm or below, a high polishing performance can be obtained from immediately after the retainer ring 8 has been attached to the holding head 4 of the CMP apparatus 1, or after a break-in polish has been given for a short time. Specifically, the polish pad 3's polish surface 3 a is flattened and fined down desirably (up to a high level) with the retainer ring 8's pressure surface 8 a. Thereby, compared with the case where the surface roughness Ra exceeds 0.2 μm, polish scrap is far more unlikely to adhere to the surface W1 to be polished of the wafer W, or scratches or the like are even more unlikely to be produced thereon. For example, let's assume that based on the wafer W's requirements accuracy (required specification), the number of defects (DN2) which have a size larger than 0.16 μm on the surface W1 to be polished of the wafer W should be equal to, or less than, twenty. In this case, in the retainer ring 8, if the pressure surface 8 a's surface roughness Ra is 0.4 μm, then 250 or more dummy wafers need polishing. In the same way, if the surface roughness Ra is 0.3 μm, 100 or more dummy wafers need polishing. In contrast, if the surface roughness Ra is 0.2 μm or below, all you have to do is merely to polish ten or fewer dummy wafers.

As a result, the time and labor taken for such a break-in polish can be kept down to the minimum. Effectively in practice, this makes it possible to enhance the production availability of the wafer W. Besides, the time taken for a break-in polish comes to the minimum, so that polish scrap produced by giving the break-in polish to dummy wafers can be reduced, scratches on the wafer W by such polish scrap are still less likely, or impurities are much less likely to adhere to the wafer W. Consequently, the production quality of the wafer W becomes higher and more stable, thus decreasing the number of defective products and enhancing the productivity further.

As described above, in the retainer ring 8, if the pressure surface 8 a's surface roughness is set to a center-line average roughness of 0.2 μm or under, the time taken for a break-in polish can be kept down to the minimum. Incidentally, if the surface roughness is set to a center-line average roughness of 0.01 μm or under, the time taken for a break-in polish can be further minimized. Specifically, as shown in FIG. 4, if the pressure surface 8 a's surface roughness Ra is 0.2 μm, for example, the defect number DN1 after ten dummy wafers have been polished is twenty, while if the surface roughness Ra is 0.01 μm or below, the defect number DN1 becomes equal to, or less than, ten. Hence, as the wafer W's requirements accuracy becomes higher, even if the number of defects (DN2) which have a size larger than 0.16 μm on the surface W1 to be polished of the wafer W should be ten or below, then in the retainer ring 8 having a surface roughness Ra of 0.01 μm or under, the requirements accuracy can be obtained simply by polishing ten or fewer dummy wafers.

Incidentally, in this embodiment, the retainer ring 8 is made of PPS, but it may also be another kind of engineering plastics, such as PEEK (polyether etherketone), PET (polyethylene terephthalate), POM (polyacetals) and PI (polyimide). Even though it is made of another such kind, if the pressure surface 8 a's surface roughness is set to a center-line average roughness of 0.2 μm or under, then as a matter of course, the time taken for a break-in polish can be restrained to the minimum. Furthermore, in this embodiment, the retainer ring 8 is made of PPS and has a single-body structure (a single-layer structure). However, it may also have a double-layer structure, for example, a double-layer structure retainer ring can be used which is formed by placing a first ring made of stainless steel and a second ring made of engineering plastics on top of each other. In this retainer ring, if the pressure surface of the second ring located at the lower layer is set to have a center-line average roughness of 0.2 μm or below, the time taken for a break-in polish can be kept down to the minimum. Moreover, in such a double-layer structure retainer ring, needless to say, the first ring and the second ring may also be fastened together with a bolt, or they can also be joined up using an adhesive agent.

The previous description is provided to enable any person skilled in the art to practice the various embodiments described therein. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments. Thus, the claims are not intended to be limited to the embodiments shown herein, but is to be accorded the full scope consistent with the language claims, wherein reference to an element in the singular is not intended to mean “one and only one” unless specifically so stated, but rather “one or more.” All structural and functional equivalents to the elements of the various embodiments described throughout this disclosure that are known or later come to be known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the claims. Moreover, nothing disclosed herein is intended to be dedicated to the public regardless of whether such disclosure is explicitly recited in the claims. No claim element is to be construed under the provisions of 35 U.S.C. §112, sixth paragraph, unless the element expressly recited using the phrase “means for” or, in the case of a method claim, the element is recited using the phrase “step for.” 

1. A CMP-apparatus retainer ring which, in a CMP apparatus that includes a polish pad disposed on a base and a holding head holding a wafer and pressing the wafer against the polish pad and that polishes the wafer chemically and mechanically: is provided inside of the holding head; has a ring shape so as to surround the periphery of the wafer; and presses the polish surface of the polish pad, characterized in that: a surface roughness of a pressure surface for pressing the polish surface of the polish pad is a center-line average roughness of 0.2 μm or below.
 2. The CMP-apparatus retainer ring of claim 1, wherein the ring is comprised of an engineering plastic material.
 3. The CMP-apparatus retainer ring of claim 2, wherein the engineering plastic material is polyphenylene sulfide (PPS), polyether etherketone (PEEK), polyethylene terephthalate (PET) or polyacetals (POM) and polyimide (PI). 